If given the values -1, 5, 2 in this sequence, what would be the missing term to make this a geometric series?

Also, what would the sum of this geometric series be?

What geometric series are you talking about? If you mean that a sequence starts -1, 5, 2, ... , that is NOT a geometric sequence: a geometric sequence is either always increasing (if the constant ratio is larger than 1) or decreasing (if it is less than 1).